Circuit boards generally have an electronic circuit including wiring lines, electrodes, switching elements and the like on a board, and are used as basic components of electronic devices such as display devices. An active matrix substrate which is a sort of circuit boards includes a plurality of wiring lines, a plurality of pixels, TFTs which are switching elements corresponding to pixels, and the like. The active matrix substrate is widely used in the production of, for example, liquid crystal display devices and EL (Electroluminescence) display devices which are used in a cellular phone, a PDA (personal digital assistant), a television receiver, and the like.
Hitherto, as a TFT included in the active matrix substrate, TFTs in which an amorphous silicon layer made of amorphous silicon are used as an active layer (hereinafter, referred to as an “amorphous silicon TFTs”) have been widely used.
In recent years, as new semiconductor alternatives to amorphous silicon, metal oxide semiconductors such as a Zn—O based semiconductor (ZnO semiconductor) and an In—Ga—Zn—O based semiconductor (IGZO semiconductor) have been considered, and TFTs (oxide semiconductor TFTs) in which an oxide semiconductor layer made of these metal oxide semiconductors is used as an active layer have been proposed. Non Patent Literature 1 discloses an oxide semiconductor TFT having characteristics in which the mobility (saturation mobility) is approximately 5.6 cm2/Vs to 8.0 cm2/Vs and the threshold value is approximately −6.6 V to −9.9 V. On the other hand, the mobility of a general amorphous silicon TFT is ordinarily 1 cm2/Vs or less.
As seen from the above, since the mobility of an oxide semiconductor TFT is higher than the mobility of an amorphous silicon TFT, the use of an oxide semiconductor TFT has a merit of enabling the creation of a more multi-functional circuit board. For example, in the active matrix substrate in which the oxide semiconductor TFT is used, even when a gate driver (scan signal line driver) is formed integrally (monolithically) on a circuit board, such a large occupation area as in an active matrix substrate in which the amorphous silicon TFT is used is not required.
Amorphous silicon TFTs in the related art can be created by using, for example, a four-mask process disclosed in Patent Literature 1. In this four-mask process, halftone exposure technology is used, a resist pattern film having two kinds of film thickness is formed on a source metal layer for forming a source electrode, a drain electrode, a data signal line and the like, and the source metal layer and a semiconductor layer located thereunder are processed using substantially the same resist pattern film.